Electrodes and sensors having nanowires

ABSTRACT

Disclosed are various embodiments for electrodes and sensors having nanowires. According to an embodiment as described, a dry sensor is provided. Nanowires, such as silver nanowires, are positioned within a polymer material, such as polydimethylsiloxane (PDMS) to form an electrode. A conductive element is attached to the electrode during its formation. Example conductive elements include, but are not limited to, a contact or a wire that may be communicatively coupled to medical equipment.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This invention claims the benefit of and priority to co-pending U.S.Provisional Patent Application No. 61/976,086 entitled “ELECTRODES ANDSENSORS HAVING NANOWIRES AND ASSOCIATED METHODS,” filed on Apr. 7, 2014,which is hereby incorporated by reference herein in its entirety.

GOVERNMENT RIGHTS NOTICE

This invention was made with government support under grant numberEEC-1160483, awarded by the National Science Foundation (NSF). TheGovernment has certain rights in the invention.

BACKGROUND

A rising interest in continuous personal health monitoring has drawnattention to the bioelectrodes currently in use, such as inelectrocardiograms (ECG), electromyograms (EMG), andelectroencephalograms (EEG), and the issues associated with them. Thesilver/silver chloride (Ag/AgCl) pre-gelled electrodes can be reliableand cost effective, however the required use of an electrolytic gellimits the long term use. For example, the gel dries out over timecausing skin irritation and a degradation in signal quality. Dryelectrodes, however, are limited by high skin-electrode impedance, poorsignal quality, durability, and complex fabrication processes which canlead to a high cost of manufacturing.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, with emphasis instead being placed uponclearly illustrating the principles of the disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1A is a schematic diagram showing an example fabrication processand drawing of an example AgNW/PDMS electrode having a snap according tovarious embodiments.

FIG. 1B is a schematic diagram showing an example fabrication processand drawing of an example AgNW/PDMS electrode having a lead wireaccording to various embodiments.

FIG. 2 shows a graph of line resistance of an example AgNW/PDMSelectrode when strained to 80% according to various embodiments.

FIG. 3 shows a graph of skin-electrode impedance trend with varyingapplication pressures according to various embodiments.

FIG. 4 shows a stationary ECG taken with an Ag/AgCl electrode and anexample AgNW/PDMS electrode according to various embodiments.

FIG. 5A shows a fabrication process flow of an example capacitivemultifunctional sensor according to various embodiments.

FIG. 5B shows a cross-sectional view of the example capacitivemultifunctional sensor of FIG. 5A according to various embodiments.

FIG. 6 shows schematic diagrams depicting three sensing modalities ofthe example capacitive multifunctional sensor according to variousembodiments.

FIG. 7A shows a relative capacitance change ΔC/C₀ versus tensile strainfor stretching and releasing of a fabricated capacitive sensor accordingto various embodiments.

FIG. 7B shows a relative capacitance change ΔC/C₀ versus tensile strainfor two measurements of a fabricated capacitive sensor, the secondmeasurement being conducted after the sensor is stretched and released100 cycles according to various embodiments.

FIG. 7C shows a one pixel sensor on a thumb joint according to variousembodiments.

FIG. 7D shows a relative capacitance change and strain associated withthumb flexure when holding a thumb up, to making a fist, and back to arelaxed state according to various embodiments.

FIG. 7E shows a schematic diagram of the patellar reflex experimentaccording to various embodiments.

FIG. 7F shows a change in capacitance and strain caused by knee motionin patellar reflex according to various embodiments.

FIG. 7G shows a relative capacitance change and strain versus time forvarious human motions according to various embodiments.

FIG. 8A shows a relative capacitance change ΔC/C₀ of one pixel versusnormal pressure for two consecutive measurements according to variousembodiments.

FIG. 8B shows a response of a pressure sensor to water dropletsaccording to various embodiments.

FIG. 8C shows a sensor array with a PDMS mold in the shape of “I” andthe resulting map of capacitance change according to variousembodiments.

FIG. 9A shows a change in capacitance for one pixel before and afterrepeated finger touching according to various embodiments.

FIG. 9B shows a change in capacitance for strong finger press (pressingmode) and gentle finger touch (proximity mode) according to variousembodiments.

FIG. 9C shows a capacitance change as a function of the distance betweensensor and finger when the finger pad approaches and leaves the sensoraccording to various embodiments.

FIG. 9D shows a capacitance change as a function of the distance betweensensor and finger when the fingertip approaches and leaves the sensoraccording to various embodiments.

FIG. 10A shows a fabrication procedure for an AgNW/PDMS flexible patchantenna according to various embodiments.

FIG. 10B shows a schematic diagram for a microstrip patch antennaaccording to various embodiments.

FIG. 10C shows a schematic diagram for a 2-element patch array accordingto various embodiments.

FIG. 11A shows a simulated radiation pattern for a AgNW/PDMS microstrippatch antenna according to various embodiments.

FIG. 11B shows a simulated radiation pattern for a AgNW/PDMS 2-elementpatch array according to various embodiments.

FIG. 12 shows a comparison of a measured and a simulated reflectioncoefficient for the AgNW/PDMS microstrip patch antenna according tovarious embodiments.

FIG. 13 shows a comparison of a measured and a simulated reflectioncoefficient for the AgNW 2-element patch array according to variousembodiments.

FIG. 14 shows a graph of an estimated and a simulated radiationefficiency for the microstrip patch antenna according to variousembodiments.

FIG. 15A shows a measured and simulated normalized radiation pattern inE-plane according to various embodiments.

FIG. 15B shows a measured and simulated normalized radiation pattern inH-plane for the microstrip patch antenna at frequency of 2.92 GHzaccording to various embodiments.

FIG. 16A shows a measured frequency response of reflection coefficientfor the AgNW/PDMS microstrip patch antenna under tensile strains from 0%to 15% according to various embodiments.

FIG. 16B shows a comparison of a simulated and a measured resonantfrequency during stretch and release and measured resonant frequencyduring stretch and release for the AgNW/PDMS microstrip patch antennaunder tensile strain from 0% to 15% according to various embodiments.

FIG. 17A shows photographs of a stretchable microstrip patch antennacomposed of AgNW/PDMS flexible conductor in a relaxed state according tovarious embodiments.

FIG. 17B shows photographs of a stretchable microstrip patch antennacomposed of AgNW/PDMS flexible conductor in a bent state according tovarious embodiments.

FIG. 17C shows photographs of a stretchable microstrip patch antennacomposed of AgNW/PDMS flexible conductor in a twisted state according tovarious embodiments.

FIG. 17D shows photographs of a stretchable microstrip patch antennacomposed of AgNW/PDMS flexible conductor in a rolled state according tovarious embodiments.

FIG. 18 shows a reflection coefficient versus frequency according tovarious embodiments.

FIG. 19 is a schematic diagram of an example RFID according to variousembodiments.

FIG. 20A shows an example planar inductor based on AgNW/PDMS stretchableconductors according to various embodiments.

FIG. 20B shows a real impedance measured from the antenna as a functionof frequency according to various embodiments.

FIG. 20C shows a Lorentzian fits of four representative impedancespectra of the LC sensor under various strains according to variousembodiments.

FIG. 20D shows a resonant frequency of the LC sensor as a function oftensile strain applied on the planar inductor according to variousembodiments.

FIG. 21 is a flowchart illustrating one example of a method employed tofabricate an electrode according to various embodiments.

FIG. 22 is a flowchart illustrated one example of a method employed tofabricate a stretchable patch antenna or a capacitive multifunctionalsensor according to various embodiments.

DETAILED DESCRIPTION

The present disclosure relates to electrodes and sensors havingnanowires. With the recent progress of robotic systems, prosthetics andwearable medical devices, efforts have been devoted towards realizationof highly sensitive and skin-mountable sensors. Sensors with varioussensing capabilities could help the robotics and prosthetic devicesmimic how a real-world object “feel” during interactions, obtainbiosignals such as finger touching and body motions sent from human, andprovide feedback information during actuating. Besides, wearable sensorsthat can be embedded into clothes or directly wrap around non-planar andbiological surfaces are widely used to monitor human body motions andoffer new opportunities for real-time health/wellness monitoring. Forthose applications, stretchability of the sensors are generally requiredin addition to flexibility. Wearable wireless communication is importantto convey sensory data and provide remote diagnosis, and a radiofrequency antenna is a critical component for the wirelesscommunication.

Antennas are conventionally fabricated by printing or etching metalpatterns on rigid substrates, which can easily crease and even fail tofunction properly when subjected to mechanical deformation (e.g.,stretching, folding or twisting). Thus, development of flexible,stretchable, and conformal antennas calls for new electronic materialsand/or new device configurations.

In accordance with embodiments as described herein, a multifunctionalwearable sensor may be formed using highly conductive and stretchablesilver nanowires (AgNWs) conductors, which enable the detection ofstrain (up to 50%), pressure (up to ˜1.2 MPa) and finger touch on asingle platform. The sensors exhibit large stretchability, highsensitivity, fast response time (˜40 ms), and good pressure mappingfunction. Such sensors can be readily mounted onto a human body tomonitor the skin strain associated with thumb flexing, knee jerk, andother human motions including walking, running, jumping, and squatting.In an example, the present subject matter may be applied as abioelectronic electrode. For example, the electrode may be used inmedical equipment for EMG, ECG, EEG, hydration sensing, musclemonitoring, and impedance-measurement. Additional details of these usesare provided herein.

In accordance with embodiments, disclosed herein is a class ofmicrostrip patch antennas that are stretchable, mechanically tunable andreversibly deformable. The radiating element of the antenna consisted ofa highly conductive and stretchable material with AgNWs embedded in thesurface layer of an elastomeric substrate. More specifically, a 3-GHzmicrostrip patch antenna and a 6-GHz, 2-element patch array can befabricated. Since a resonant frequency increases with increasing tensilestrain, the antenna can be used for wireless strain sensing. Finally,the antennas maintain the same spectral properties under severe bending,twisting, and rolling.

Electrodes and sensors having nanowires and associated methods aredisclosed herein. According to an aspect, a method of producing a sensoris provided. The method includes positioning nanowires, such as AgNWs,within a polymer material, such as polydimethylsiloxane (PDMS), to forman electrode. The method also includes attaching a conductive element tothe electrode. Example conductive elements include, but are not limitedto, a contact, a button, and a wire. In an example, a conductive elementmay be attached by use of a liquid metal. The metal in this example maybe liquid at room temperature for use in reinforcing the contact. Anexample metal includes, but is not limited to, eutectic gallium indium(EGaIn).

According to another aspect, a method of producing a capacitive sensoris provided. The method includes positioning a first plurality ofnanowires within a first polymer material. The method also includespositioning a second plurality of nanowires within a second polymermaterial. Further, the method includes attaching a non-conductivematerial to a side of the first polymer material and to a side of thesecond polymer material.

In accordance with embodiments, a dry silver nanowire-based electrode isemployed for use in electrocardiogram (ECG or EKG) equipment,electroencephalogram (EEG) equipment, electromyogram (EMG) equipment,impedance-measurement equipment and other related applications (e.g.,clinical applications and long-term health monitoring applications).Silver nanowires (AgNWs) can be embedded or otherwise positioned inpolydimethylsiloxane (PDMS) to create a highly conductive stretchableand flexible network. PDMS can be used in biomedical applications due toits nontoxicity and high permeability to water and gas. Silver can beused in biomedical applications due to its antibacterial properties.

To fabricate a dry electrode, AgNWs can be cast onto a silicon substrateor suitable substrates, such as plastic and glass. After the solventcompletely evaporates, a network of AgNWs remains and liquid PDMS ispoured over the nanowires. At this stage, either a conductive element,such as a lead wire or a steel snap, is pressed on top of the AgNW/PDMSmixture. The substrate is then placed in vacuum to remove air bubblesfrom the PDMS and then cures in an oven at 100° C. for 1 h. When thecured PDMS is peeled off the substrate, the AgNW network is visiblybonded to the PDMS and the lead wire or snap are securely connected tothe AgNW/PDMS network. A schematic diagram of the fabrication processfor an electrode with a lead wire and an electrode with a snap is shownin FIG. 1. Velcro straps can be added to the electrodes to allow for theelectrodes to be worn on the wrist.

An example benefit to using AgNWs embedded in PDMS is their ability tomaintain high conductivity even when stretched. The surface resistanceof the electrode can be measured in-situ as the electrode is strainedfrom 0-50%. The resistance of the electrode stabilizes after repeatedstretching and releasing meaning that the electrode will give consistentreadings under varying strain conditions.

An issue with surface bioelectrodes is the skin-electrode contact. Theskin's electrical properties are highly variable which leads to issuesin acquiring consistent ECG readings and other similar readings. At lowfrequencies, the impedance of the skin is determined by the stratumcorneum, the outermost layer of skin. While Ag/AgCl electrodes have agel to help moisten this layer and improve electrode-skin contact, dryelectrodes eliminate the use of this gel. Therefore, dry electrodes needa low skin-electrode impedance to attain signals of comparable qualityto the Ag/AgCl electrodes. In addition, low skin impedance can helpreduce motion artifacts, which are discrepancies in the ECG signalcaused by movement. The skin-electrode impedance can be measured byperforming a frequency sweep from 40 Hz-100 kHz using an impedanceanalyzer.

Applying pressure on the electrode can affect the resulting impedancetrend. Although the impedance decreases with increasing applicationpressure, the benefits gained from increasing the pressure past acertain point are minimal. A force sensor can be used to measure theapplication pressure of the electrode on the skin. Three differentpressure applications can be tested, including light (0.11 psi), medium(0.27 psi), and hard (1.72 psi). A medium pressure can be used whenapplying the electrodes for ECG tests as it may be the most comfortablefor the subject while still applying enough pressure to obtain good ECGsignals.

When taking an ECG with an Ag/AgCl electrode, a series of steps toremove the upper layer of the stratum corneum are followed consisting ofremoving the dead cells by abrasion and applying an extra electrolyticgel to hydrate the skin before applying the pre-gelled Ag/AgClelectrode. A benefit of a dry electrode is to minimize skin preparation,so no skin preparation is required before applying the AgNW/PDMSelectrode for ECG testing. ECG signals can be measured using an ECGamplifier while the subject is resting. These signals can be taken withthe AgNW/PDMS dry electrodes in the Lead I position and with commercialelectrodes also in the Lead I position.

The resistance of the electrode, shown in FIG. 2, increased linearlywith increasing strain and stabilized to approximately 8Ω after repeatedstretching. The resistance value can be tailored by changing the densityof the nanowires. This indicates that the AgNW/PDMS electrode canperform consistently under varying strain conditions, making it idealfor use in wearable or continuous health monitoring. Theimpedance-pressure trend is shown in FIG. 3. As the application pressureincreases, the peak impedance decreases. This can be attributed to anincreased contact surface area between the skin and electrode. Theskin-electrode impedance values acquired are high, but do not affect thequality of the ECG signal.

ECG signals can be acquired using the AgNW/PDMS electrodes and comparedwith signals acquired using conventional Ag/AgCl pre-gelled electrodes,as shown in FIG. 4. The signals were taken with the electrodes in theLead I position, with the negative electrode placed on the right arm,the positive electrode placed on the left arm, and the ground electrodeplaced on the right leg. When acquiring ECG signals with the Ag/AgClelectrodes, the skin was cleaned and a small amount of electrolytic gelwas added to the pre-gelled electrodes. However, no skin preparation orelectrolytic gel was used when attaining ECG signals with the AgNW/PDMSelectrodes. The P wave, QRS complex, and T wave show clearly in each ofthe ECG signals. These components are used for diagnostic purposes, soit is imperative that the waves can be viewed clearly. No criticaldifferences between the AgNW/PDMS signal and the Ag/AgCl signal exist.When the extra electrolytic gel was not used with the pre-gelledelectrodes, the signal acquired was of poorer quality than the AgNW/PDMSsignal.

A silver nanowire based electrode for use in long-term ECG monitoring,EKG monitoring, or other suitable applications, can be fabricated. TheAgNW/PDMS electrodes can be characterized by conductivity as a functionof strain and by the electrode-skin impedance as a function of frequencyand pressure. The conductivity of the AgNW/PDMS electrodes is retainedthroughout multiple 0-50% strains which show that the electrodes canmaintain a high performance level in different strain/flex conditions.As expected, the initial impedance of the skin-electrode interfacedecreased with increasing application pressure. Although highskin-electrode impedance usually leads to a low-quality ECG signal, theAgNW/PDMS electrode yielded a high quality signal even though theskin-electrode impedance is high. The electrodes were then connected toan ECG amplifier to verify the ability to acquire high quality ECGsignals. The ECG performance of each design of the AgNW/PDMS electrodescan be compared to conventional, pre-gelled Ag/AgCl electrodes and foundto yield comparable results that can be used for diagnostic andmonitoring purposes. The AgNW/PDMS electrode that was fabricated with asnap or contact was successfully connected to the ECG amplifier usingconventional lead wire connections which will allow for use withconventional ECG machines already in existence. Further, the AgNW/PDMSelectrode fabricated with a lead wire can be connected to lab-made ECGmachines or to conventional ECG machines via the use of an alligatorclip.

As disclosed herein, a dry electrode alternative to the widely-used,pre-gelled Ag/AgCl electrodes can be successfully fabricated and proveto be a viable choice for use in ECGs in both the clinical andcontinuous health monitoring settings. The AgNW/PDMS electrodes can befabricated using an inexpensive method that has the potential to scaleup to a large manufacturing assembly. The electrode design gives ECGsignals of comparable quality to the Ag/AgCl electrodes without the skinpreparation required of using the Ag/AgCl electrodes. The elimination ofthe electrolytic gel can allow for the AgNW/PDMS electrode to be wornfor long periods of time without irritating the skin. The dry electrodedesign is compatible with current ECG equipment and will allow theelectrodes to be easily integrated into existing biomedical devices athospitals and clinics. The robust design of the AgNW/PDMS electrode canallow for reusability and will also allow it to be used in long-termmonitoring.

Initially, AgNW suspension in ethanol or other suitable solvents (e.g.,water) can be drop-casted onto a pre-cleaned substrate and the metal NWscan then be dried to form a uniform and conductive coating of NWs. SuchAgNW conductors can be patterned through a pre-patterned PDMS shadowmask, for example, with line width of ˜2 mm and spacing of ˜2 mm (Step 1in FIG. 5A). Liquid PDMS (mixing the “base” and the “curing agent” witha weight ratio of 10:1) can then be casted onto the Silicon (Si)substrate that included the AgNW conductors on top, and cured at 65° C.for 12 hours. As a result, all the patterned AgNW conductors can beembedded just below the PDMS surface when it is peeled off the Silicon(Si) substrate (Step 2 in FIG. 5A). Eutectic gallium-indium (EGaIn,Aldrich, ≧99.99%) liquid metal can be applied to the two ends of theAgNW/PDMS strips to serve as conformal electrodes. After that, theAgNW/PDMS film can be positioned orthogonal to another identicalAgNW/PDMS film face-to-face (Step 3 in FIG. 5A).

A soft dielectric layer (e.g., Ecoflex silicone elastomer) can beintroduced as the dielectric layer of the capacitors. Liquid Ecoflexmade by mixing part A and part B with the ratio of 1:1 can be appliedbetween the two orthogonally positioned AgNW/PDMS films. At the sametime, copper wires can be embedded inside the liquid metal and coveredby Ecoflex liquid. Finally, the whole structure can be degassed in avacuum oven followed by curing under ambient condition for approximately4 hours (Step 4). This way, the Ecoflex layer can be sandwiched betweenthe orthogonally patterned stretchable AgNW conductors to form thecapacitive sensors. The AgNW, polydimethylsiloxane (PDMS) and dielectriclayer as shown in FIG. 5B are about 5 μm, 0.2 mm and 0.5 mm inthickness, respectively, although other sizes can be implemented. Invarious embodiments, an individual capacitive sensor can be fabricatedfollowing the same process.

The capacitance was measured by an AD7152 capacitance-to-digitalconverters evaluation board. The principles for strain sensing, pressuresensing and touch sensing to be discussed later are schematically shownin FIG. 6.

FIG. 7A shows the relative capacitance change ΔC/C₀ versus tensilestrain during stretching and releasing. As the sensor is uniaxiallystretched, the length (along the strain direction) of the electrodeincreases, while the width of the electrode and the separation betweenthe two electrodes decrease, resulting in an increase in capacitance.The strain sensor exhibited good linearity and reversibility up to verylarge strain level (50%). In addition, the sensor exhibits excellentstability, as shown in FIG. 7B. The gauge factor (the relative change incapacitance divided by the mechanical strain) was found to be ˜0.7.Strain sensors based on resistive mechanism usually suffer from largehysteresis and nonlinearity under large strain. For the capacitivesensors described herein, the hysteresis was found to be negligible. Inaddition, the sensors can reliably detect the strain below 1%.

The strain range during human movement can be much larger than that ofconventional strain gauges. In FIGS. 7C-7G, it is demonstrated throughreal-time strain measurements during large movements that theskin-mountable sensors as described herein can help monitor the bodymotions, which provides important information for feedback control inrobotic systems, prosthetic devices, and other suitable uses. At thesame time, the sensors can be beneficial for continuous health andwellness monitoring, for example, to help detect physiologicalconditions (such as knee-jerk) for diagnoses, monitor body motionsduring rehabilitation, and quantize the body movement to evaluate anathlete's performances.

In various embodiments, a matrix of capacitors, such as a 7×7 array ofcapacitors (“pixels”), can be fabricated following the process shown inFIG. 5A to form a pressure sensor that has spatial resolution. When apressure is applied on the capacitor, the separation between the twoAgNW layers decreases, leading to an increase in capacitance (as shownin FIG. 6B). The relative capacitance change ΔC/C₀ in FIG. 8A shows abilinear dependence on the pressure. Compared with previously reportedcapacitive pressure sensors, the sensitivity of the sensors is higherthan those with carbon nanotube electrodes (0.23 MPa-1 over the pressurerange up to ˜1 MPa) and those with serpentine gold electrodes (0.48MPa-1 over the pressure range up to 0.25 MPa), both using Ecoflex asdielectric layers. Pressure sensors composed of copper electrodes and anair gaps encapsulated by PDMS showed nonlinear response with asensitivity of 3%/mN (4.8 MPa-1) over the range of 40 mN (250 kPa). Thesensors with copper clad laminated composites on unstretchable polyimidesubstrate exhibited a sensitivity of 9.2 MPa-1 for the range of 40 kPa.Highly sensitive pressure sensors using microstructured PDMS dielectriclayer and PET substrate can be achieved. The sensor showed similarbilinear response (0.55 kPa-1 for less than 2 kPa and 0.15 kPa-1 for 2-7kPa), but on non-stretchable polyester substrate.

Fast response time is important in realizing real-time pressuremonitoring. Small loadings can be applied by dispersing three 0.06 gwater droplets, as shown in FIG. 8B. Here, the response time (rise time)can be defined as the time interval between 10% and 90% of the steadystate values. Response time of the sensors, as described herein, wasestimated to be around 40 ms, which is much shorter than those reportedfor other pressure sensors such as the flexible polymer foam basedcapacitive sensor (several seconds) and the one using an Gold (Au) filmpatterned on a PDMS membrane (200 ms). Very few pressure sensors cansimultaneously achieve the large stretchability, fast response, highsensitivity, and good linearity. To demonstrate the function ofmeasuring the spatial distribution of pressure, a 2.7 g mold with theshape of letter “I” was cut and placed onto the sensor. The resultingrelative capacitance changes are plotted in FIG. 8C, where brightnesscorresponds to a higher capacitance change. As can be seen from FIG. 8C,the spatial distribution of the applied pressure is clearlyidentifiable.

The capacitive sensors can also be used to detect finger touch and/orthe touch of another grounded conducting medium, due to the partiallygrounded electric field by finger, as shown in FIG. 6C. The sensor canfunction well in both situations: (1) proximity mode (the finger isapproaching, no force applied) and (2) pressing mode (force is applied).FIG. 9A shows the response of one pixel in the sensor array to fingertouch (no force applied). As expected, the capacitance decreases uponfinger touching. In order to probe the determining factors of thecapacitance change, we approached the sensor from 30 cm away untiltouching with different finger areas. FIGS. 9C and 9D reveal that largerinteraction area and shorter distance lead to a larger capacitancedecrease because of the increased portion of the electric fieldintercepted by the finger.

In some touch sensing applications, forces from finger touch areinevitable. FIG. 9B presents the results for strong finger press (largeforce applied) and gentle finger touch (no force applied). The resultindicates that the sensor can be reliably used as a touch sensor undereither proximity mode or pressing mode (e.g., gentle or strong touches).For strong press, the interacting area between the finger and the pixelelectrode is typically larger compared to gentle touch, which leads to amuch larger decrease in capacitance. Flexible resistive andpiezoelectric touch sensors have strechability limited either by thesensing layer or substrate material. Moreover, these sensors can only beused when the finger presses the sensors. In contrast, the capacitivetouch sensors have a longer detecting range. This characteristic may bevery useful in applications where contact between an electronic deviceand a human should be avoided or when contact between an electronicdevice and a human is undesirable.

By using stretchable materials for pressure sensors, the existence oftensile strain and normal pressure can be distinguished from thedistribution of capacitance changes. Tensile straining affects all thepixels along the strain direction; in contrast, pressure only affectsthe pixels in the immediate vicinity of the load. The existence of thefinger touch can also be identified and distinguished because onlyfinger touch causes the decrease in capacitance. According to thespecific needs, all the pixels can have the three functions or differentpixels can be engineered to have different localized functions.

In various embodiments, the stability, sensitivity, linearity, detectingrange, and/or response time can be further enhanced via optimization ofgeometry and materials. Further, the multifunctional sensors can beintegrated with wearable devices (e.g., sensors, actuators, antennas,and power devices) and used as the conformal intelligent surfaces tointeract with human and the environments in robotic systems,prosthetics, wearable health monitoring devices, or flexible touch pads.

Moving on to FIG. 10A, shown is a fabrication procedure for theAgNW/PDMS patch antennas. Two types of patch antennas—the single patch(FIG. 10B) and 2-element array (FIG. 10C)—can be fabricated using thesame or a similar process. The thickness of the AgNW/PDMS layer can be˜0.5 mm and the separation between the radiating element and the groundplane can be ˜1 mm (±0.1 mm).

In various embodiments, the single patch antenna consists of arectangular radiating patch, a ground plane, and a uniform layer ofdielectric substrate between them. Dimensions of the patch can bedesigned based on a transmission-line model, which gives the width W andthe length L as functions of the resonant frequency f_(res), therelative permittivity of substrate material ∈_(r), and the thickness ofsubstrate h. The substrate material PDMS has a reported relativepermittivity ranging from ∈_(r)=2.67 to 3.00 and loss tangent rangingfrom tan δ=0.01 to 0.05 over operating frequency range of 1.0 GHz to 5.0GHz. Accordingly, the substrate material can be modeled with a relativepermittivity of ∈_(r)=2.80 and a loss tangent of tan δ=0.02 for a 3-GHzapplication.

Conductivity of the AgNW/PDMS stretchable conductor is ˜8,130 Scm-1before stretching. Here, a constant conductivity of 8,130 Scm-1 can beused for the antenna considering the applied strains can be relativelysmall. To obtain the resonance frequency of 3 GHz, the rectangular patchwas designed to be 36.0 mm×29.2 mm, backed with a 45.0 mm×40.0 mm groundplane, although other suitable sizes can be implemented. To match theinput impedance with a 2.5 mm×8.0 mm 50Ω microstrip feed line, the insetfeeding method was employed, which left a 3 mm external part andeliminated the need for an external matching network. Length and widthof the cutout inset region were optimized in ANSYS HFSS to achieve lowerreturn loss and less additional coupling between patch and feed line.

The 6-GHz 2-element array patch antenna can be designed similarly withthe same material parameters except an increased loss tangent of tanδ=0.05. Two identical radiating elements with dimensions shrinking to18.0 mm×14.3 mm can be arranged in parallel, and fed simultaneously by afeeding network. Since the doubled operating frequency renders the inputimpedance more sensitive to inaccuracy in dimensions, the matchingstrategy can be changed by introducing an impedance transformer at theedge of each radiating element to reduce possible fabrication error.Note that due to fabrication error, the obtained resonance frequenciesfor the patch and the 2-element array are 2.92 and 5.92 GHz,respectively.

Simulated radiation patterns of the one and two element antennas wereobtained by far-field calculation in ANSYS HFSS, as shown in FIGS.11A-11B. Simulation results for the radiation properties of bothantennas are summarized in Table 1 for comparison. The 2-element array,compared with the single element, increases the directivity by 4.5 dBand the fractional bandwidth by 2.5%, with higher radiation efficiencyat the same time.

TABLE 1 Comparison of Simulated Radiation Properties for MicrostripPatch Antenna and 2-Element Patch Array Radiation Properties MonopolePatch 2-Element Patch Array Resonant Frequency 2.92 GHz 5.92 GHz PeakDirectivity 4.16 dBi 8.14 dBi Peak Gain 0.37 dBi 4.90 dBi RadiationEfficiency 41.83% 48.83% Bandwidth 88 MHz (3.0%) 330 MHz (5.5%)

The patch antennas were characterized experimentally and compared to thesimulated results. Measured and simulated frequency responses agreedvery well, with the difference within the manufacturing imperfection andmeasurement uncertainty. FIG. 12 shows the measured spectrum response ofthe reflection coefficient over frequency range of 2 GHz to 4 GHz forthe single patch antenna before stretching, with S11 below −20 dB at thecenter frequency of 2.92 GHz and above −1 dB far outside the operatingregion. The bandwidth, defined as the frequencies where S11<−10 dB, was97.5 MHz.

The simulated reflection coefficient for the 2-element array wascompared to the measured reflection coefficient from 4 GHz to 8 GHz. Thearray was initially designed with a single operating band at 6 GHz whilethe measured results showed an additional operating band at around 5.3GHz. The mechanism of the unexpected resonance was studied byintroducing small dimension deviations due to possible fabricationerrors compared to the antenna model with ideal dimensions. As is shownin FIG. 13, when we assume that one of the radiating elements wasfabricated larger in length than the other, the simulated frequencyresponse would extend to the lower frequency band and form another bandlocated at around 5.3 GHz for a 1 mm deviation in length, which was veryclose to what was observed experimentally.

Far-field performance for the single patch antenna was tested in ananechoic chamber. The peak gain over frequency range of 2 GHz to 4 GHzwas measured. Radiation efficiency was estimated using the measured gainand simulated directivity, which is compared to the simulated radiationefficiency in FIG. 14. To further study loss mechanisms of the AgNW/PDMSpatch antenna with respect to radiation efficiency, antennas composed offour combinations of dielectric substrates and metal materials weremodeled. Both AgNW/PDMS conductor with conductivity of 8130 Scm-1 andperfect electric conductor (PEC) with infinite conductivity for themetal components were considered. Also, both PDMS with loss tangent oftan δ=0.02 and lossless dielectric substrate was modeled. Table 2summarized the simulation results of radiation efficiency for all thefour combinations.

TABLE 2 Comparison of Simulated Radiation Efficiency of Antennas inDifferent Dielectric and Metal Materials Substrate Metal RadiationEfficiency Lossy PDMS AgNW/PDMS 41.53% Lossy PDMS PEC 55.76% LosslessMaterial AgNW/PDMS 67.20% Lossless Material PEC  100%

Compared to the ideal configuration, radiation efficiency was decreasedfrom 100% to around 56% by the lossy substrate, and to around 67% by theAgNW/PDMS with finite conductivity. For completeness, the radiationpattern for the antenna in E-plane and H-plane is shown in FIG. 15. Thestretchable antenna exhibits excellent radiation properties as well asgood agreement with the simulated results.

To test the mechanical tunability as a stretchable antenna, tensilestrain ranging from 0% to 15% was applied to the AgNW/PDMS patch antennain the width direction (perpendicular to the cable connection), whilethe reflection coefficient was collected by the network analyzersimultaneously. The antenna was tested on a custom-made mechanicaltesting stage, where all the components are made of insulators (e.g.,ceramic, glass and Teflon). FIG. 16a shows the measured frequencyresponse of the reflection coefficient under tensile strain from 0% to5%. With the increasing strain, the spectrum response shifted to higherband, the center frequency increased almost linearly and the −10 dBbandwidth remained higher than 80 MHz, as listed in Table 3. The resultssuggest that performance of the stretchable antenna was not largelycompromised during stretching.

TABLE 3 Measured Resonant Frequency and Bandwidth of the AgNW/PDMSMicrostrip Patch Antenna Under Tensile Strains from 0% to 15% Strain (%)0 3 6 9 12 15 Resonant 2.947 2.991 3.020 3.044 3.063 3.083 Frequency(GHz) Bandwidth 239 244 253 254 243 258 (MHz)

The strain was then increased to 15% and slowly removed from theantenna. The center frequency was also measured during the releasingprocess. Upon complete release of the strain, the antenna returned toits original resonant frequency demonstrating excellent reversibledeformability.

To analyze the frequency shift due to the applied strain, we accountedfor the changing dimensions as functions of the strain. PDMS is atypical hyperelastic material where the total volume is constant duringdeformation. Therefore when the antenna is elongated in the widthdirection, the length and height shrink proportionally. The resonantfrequency f_(res) is determined by the length of the radiating patch as:

$\begin{matrix}{{f_{res} = \frac{c}{2L\sqrt{\in_{reff}}}},} & \left( {{eq}.\mspace{14mu} 1} \right)\end{matrix}$

where c is the speed of light in vacuum, L is the length of microstrippatch antenna and ∈_(reff) is the effective relative permittivity of themicrostrip, to account for the differing permittivities of the air andsubstrate material.

When a tensile strain of s is applied, the new dimensions of theantenna, patch width W, patch length L, and substrate thickness h as thefunction of s is:

$\begin{matrix}{{W = {W_{0}\left( {1 + s} \right)}},} & \left( {{eq}.\mspace{14mu} 2} \right) \\{{L = \frac{L_{0}}{\sqrt{1 + s}}},} & \left( {{eq}.\mspace{14mu} 3} \right) \\{{h = \frac{h_{0}}{\sqrt{1 + s}}},} & \left( {{eq}.\mspace{14mu} 4} \right)\end{matrix}$

where W₀, L₀, and h₀ are the original dimensions before stretch, and fora small strain s<<1, the new resonant frequency is represented as:

$\begin{matrix}{{f_{res} = {\frac{c\sqrt{1 + s}}{2L_{0}\sqrt{\in_{reff}}} \approx {\frac{c}{2L_{0}\sqrt{\in_{reff}}}\left( {1 + {\frac{1}{2}s}} \right)}}},} & \left( {{eq}.\mspace{14mu} 4} \right)\end{matrix}$

which gives a linear relationship between the resonant frequency f_(res)to the applied strain s. The effective dielectric constant ∈_(reff) wasupdated for each strain level.

Results are compared to measurements during both stretching andreleasing processes in FIG. 16b . The recorded center frequency as afunction of tensile strain agrees well with what is predicted bymodeling. The difference between simulated and measured resonantfrequencies is within ±3 MHz at each point, which is relativelyinsignificant (within ±0.2%).

The stretchable antenna is thus well suited for wireless strain sensingapplications. To further demonstrate the reversible deformability of theantenna, as described herein, the antennas can be subjected to otherdeformation modes including bending, twisting, and rolling, as shown inFIG. 17. For example, the antenna can be deformed by bending by 90°along the long axis, twisting along the long axis, rolling on both axes.After the deformed antenna is returned to its original state, itmaintains almost the same spectral properties with difference of theresonant frequencies within ±1 MHz (<0.1%) before and after deformationin each case, which demonstrates that the AgNW/PDMS flexible antenna isreversibly deformable and robust.

It has been demonstrated that a class of microstrip patch antennas thatare stretchable, mechanically tunable and reversibly deformable. A 3-GHzpatch antenna and a 6-GHz 2-element patch array were fabricated.Radiating properties of the antennas were characterized under tensilestrain, which agreed well with the simulation results. The antenna wasmechanically tunable, enabling the resonant frequency to change as afunction of the applied tensile strain. Thus it was well suited forapplications like wireless strain sensing. The radiation efficiency waslimited by losses in both the PDMS substrate and AgNW. The antennas werealso demonstrated to maintain the same spectral properties after severebending, twisting, and rolling. The material and fabrication techniquereported here could be extended to achieve other types of stretchableantennas with more complex patterns and multi-layer structures.

In various embodiments, stretchable antennas can be fabricated asfollows. AgNWs with average diameter of ˜90 nm and length in the rangeof 10-60 μm (or other suitable sizes) can be synthesized in a solution.For example, they can dispersed in ethanol with a concentration of 10mg/mL. As shown in FIG. 10a , the antenna pattern can be cut out from astencil mask on a pre-cleaned substrate such as silicon (Si) wafer.AgNWs solution was drop-casted into the mask on top of a hot plate (setat 55° C.) and then dried to form a conductive film of AgNWs with thedesired antenna pattern. After peeling off the stencil mask, liquid PDMSwas casted on top of the AgNW film, and heated at 100° C. for 1 hour tocure. When the cured PDMS layer was peeled off the substrate, the AgNWfilm was embedded into the PDMS forming a surface layer both conductiveand stretchable. Fabrication of the ground plane layer can follow thesame procedure, and then bond with the patch layer before the liquidPDMS cured.

In various embodiments, the stretchable antennas can be modeled and/ordesigned as follows. The width W and the length L are designed based onthe transmission-line model.

$\begin{matrix}{{W = {\frac{c}{2f_{res}}\sqrt{\frac{2}{\in_{r}{+ 1}}}}},} & \left( {{eq}.\mspace{14mu} 6} \right) \\{{L = {\frac{c}{2f_{res}\sqrt{\in_{reff}}} - {2\Delta \; L}}},} & \left( {{eq}.\mspace{14mu} 7} \right) \\{{\in_{reff}{= {\frac{\in_{r}{+ 1}}{2} + {\frac{\in_{r}{- 1}}{2}\left\lbrack {1 + {12\frac{h}{w}}} \right\rbrack} - \frac{1}{2}}}},} & \left( {{eq}.\mspace{14mu} 9} \right) \\{{{\Delta \; L} = {0.412h\frac{\left( {\in_{reff}{+ 0.3}} \right)\left( {\frac{W}{h} + 0.264} \right)}{\left( {\in_{reff}{- 0.258}} \right)\left( {\frac{W}{h} + 0.8} \right)}}},} & \left( {{eq}.\mspace{14mu} 10} \right)\end{matrix}$

with ΔL as the “extended” length at each end because fringing fields atthe patch edges make the length appear larger electrically thanphysically. For low frequencies (<10 GHz) the effective dielectricconstant is essentially constant, referred to as the static values andgiven by eq. 8. Eq. 9 is a common approximate relation for the extensionof length depending on the effective dielectric constant ∈_(reff) andthe width-to-height ratio (W/h). Typically, ΔL<<L.

An antenna can be connected to a coaxial cable by a SMA connector.S-parameters can be collected using an Agilent E5071C Vector NetworkAnalyzer to measure the resonant frequency and reflection coefficient.Radiation patterns were measured in the anechoic chamber at the NC StateRemote Educational Antenna Lab (REAL). 2D pattern cuts were measured inthe orthogonal E- and H-planes (YZ and XZ planes). Each cut was obtainedby rotating the antenna under test (AUT) in 10 degree increments whilerecording the received signal with a broadband horn antenna (A.H.Systems) to produce the relative pattern plot. Absolute gain wascalculated via gain comparison to a standard gain horn (A.H. Systems).The results given in the present disclosure represent the co-polarradiation patterns and gain.

Various types of strain sensors have been reported, which offerexcellent performance in terms of strain range, sensitivity, linearityand stability. However, most of them require physical connection toexternal electronics and thus potentially limit their applications onmoving objects or in sealed environment. It is therefore of interest todevelop wireless strain sensors. In accordance with embodiments, apassive wireless strain sensor is provided that follows the principle ofradio-frequency identification tag (RFID), based on our AgNW/PDMSstretchable conductors; see the schematic diagram in FIG. 19. The samepatterning technology was first employed to fabricate a planar inductoron PDMS, as shown in FIG. 2A. By connecting the planar inductor with adummy capacitor, an inductor-capacitor (LC) circuit was formed (FIG.19). The LC circuit was remotely interrogated with a loop antenna viamutual inductance coupling between the planar inductor and loop antenna.The resonant frequency of the LC circuit was determined by measuring thereal portion of the impedance spectrum across the terminals of theantennas by a precision impedance analyzer (Agilent 4294A). Tensilestrains were then applied on the AgNW/PDMS based inductor, while theimpedance spectra were recorded at the same time, from which theresonant frequency as a function of strain was extracted.

FIG. 20b shows the measured real impedance spectra in the frequencyrange of 5 to 40 MHz. The black and red curves represent the impedancespectra measured from the loop antenna with and without the LC sensorpresented, respectively. By subtracting the red spectrum (i.e.,background signal) from the black one, the intrinsic impedance of theloop antenna was removed from the signal and the net sensor response wasobtained (the green curve in FIG. 20). When a strain was applied on theplanar inductor, the resonant frequency changed in response to thechange of the inductance, which was in turn due to geometric change ofthe planar inductor under strain. The inductance was found to increasewith the tensile strain. Lorentzian fits of four representative realimpedance spectra (only the portion around the peak from 22 MHz to 27.5MHz) of the LC sensor under various strains are shown in FIG. 20c . Theoriginal data are provided in the Supplementary Information. It can beseen that the spectra shift gradually leftward with the applied tensilestrain, which means that the resonance frequency of the LC sensordecrease with the tensile strain. The resonant frequency of the LCsensor was obtained from the fits and plotted in FIG. 20d as a functionof the tensile strain. The resonant frequency of the LC strain sensordecreased monotonically with the strain up to 35.1%. The strain rangewas limited by the size of the loop antenna. A larger strain range couldbe achieved by using a larger loop antenna. Passive inductively coupledwireless strain sensors have been fabricated before; however, the strainrange was limited. With the assistance of the AgNW/PDMS stretchableconductor, a large measurement range of 0-35.1% was achieved here, whichis the largest strain range for any wireless strain sensor to the bestof our knowledge.

While the embodiments have been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments can be used or modifications andadditions can be made to the described embodiment for performing thesame function without deviating therefrom. Therefore, the disclosedembodiments should not be limited to any single embodiment, but rathershould be construed in breadth and scope in accordance with the appendedclaims.

Referring next to FIG. 21, shown is a flowchart that provides oneexample of fabricating an electrode according to various embodiments.First, a pattern may be cut to create a stencil mask, such as apre-cleaned substrate (2102). A plurality of nanowires, such as AgNWs orcarbon nanotubes, are casted on the substrate (2103) cut out in 2102. Invarious embodiments, the substrate comprises silicon, plastic, glass, orany combination thereof. After the solvent has evaporated, a network ofnanowires (e.g., AgNWs) remains and liquid PDMS is poured over thenanowires (2106) to create a mixture of nanowires and PDMS. Next, aconductive element (e.g., lead wire and/or a steel snap) is pressed ontop of the nanowire/PDMS (e.g., AgNW/PDMS mixture)(2109). In variousembodiments, the conductive element may be configured to operativelyconnect to photovoltaic equipment, a display device, and artificialskin. Similarly, in various embodiments, the conductive element may beconfigured to operatively connect to prosthetic equipment, a mechanicalmotion detector, pressure sensing equipment, a strain gauge, hydrationsensing equipment, a biomorph actuator, or an actuator.

The substrate is then placed in a vacuum to remove air bubbles from thePDMS (2112). Then, the PDMS is heated to cure the PDMS (2115). In oneexample, the PDMS is cured in an oven at a suitable temperature for asuitable length of time. In various embodiments, the PDMS is cured inthe oven at 100° C. for 1 h. Next, the cured PDMS is peeled off thesubstrate (2118), after which the AgNWs network is visibly bonded to thePDMS and the lead wire and/or the snap are securely connected to theAgNW/PDMS network. Finally, Velcro straps can be added to the electrodesto allow for the electrodes to be worn, for example, on the wrist(2121).

Referring next to FIG. 22, shown is a flowchart that provides oneexample of fabricating, for example, a stretchable antenna and/or astretchable capacitive sensor according to various embodiments. In someembodiments, nanowires (e.g., AgNWs) are suspended in ethanol or othersuitable solvents (e.g., water) (2203) to form a nanowire solution.However, in alternative embodiments, a pre-manufactured nanowiresolution may be employed. Next, a suitable pattern, such as an antennapattern or a capacitive sensor pattern, can be cut out from a stencilmask on a pre-cleaned substrate, such as a silicon (Si) wafer (2206).Next, the nanowire solution is drop-casted onto the stencil mask cutfrom the pre-cleaned substrate (2209). The solution drop-casted onto thesubstrate metal NWs are then dried to form a uniform and conductivecoating of NWs (2212).

In various embodiments, the nanowire conductors can be patterned througha pre-patterned shadow mask (e.g., a PDMS shadow mask). For example, thenanowire conductors can be patterned with a line width of ˜2 mm andspacing of ˜2 mm. Liquid PDMS can then be casted onto the substrate thatincluded the nanowire conductors on top (2215). The liquid PDMS is thencured at a suitable temperature for a suitable length of time (e.g., at65° C. for 12 hours) (2218). The cured PDMS surface is then peeled off(2221). As a result, all the patterned nanowire conductors are embeddedjust below the PDMS surface when it is peeled off the substrate.

Finally, the antenna and/or capacitive sensor can be formed by applyingtwo or more layers and/or arrangements of AgNWs/PDMS (2224). Forexample, the AgNW/PDMS film can be positioned orthogonal to anotheridentical AgNW/PDMS film face-to-face. With respect to an antenna, thefabrication procedure for the AgNW/PDMS patch antennas of FIGS. 10A-Cmay be employed. For example, two types of patch antennas−the singlepatch (FIG. 10B) and 2-element array (FIG. 10C) can be fabricated usingthe flowchart of FIG. 22. In various embodiments, the thickness of theAgNW/PDMS layer can be ˜0.5 mm and the separation between the radiatingelement and the ground plane can be ˜1 mm (±0.1 mm).

With respect to a stretchable capacitive sensor, a soft dielectric layer(e.g., Ecoflex silicone elastomer) can be introduced as the dielectriclayer of the capacitors. Liquid Ecoflex made by mixing part A and part Bwith the ratio of 1:1 can be applied between the two orthogonallypositioned AgNW/PDMS films. At the same time, copper wires can beembedded inside the liquid metal and covered by a liquid, such asEcoflex liquid. Finally, the structure can be degassed in a vacuum ovenfollowed by curing under ambient condition for approximately 4 hours orother suitable length of time. As a result, the Ecoflex layer ispositioned between the orthogonally patterned stretchable AgNWconductors to form the capacitive sensors.

Although the flowcharts of FIGS. 21 and 22 show a specific order ofexecution, it is understood that the order of execution may differ fromthat which is depicted, when feasible. For example, the order ofexecution of two or more blocks may be scrambled relative to the ordershown. Also, two or more blocks shown in succession in FIGS. 21 and 22may be performed concurrently or with partial concurrence. Further, insome embodiments, one or more of the blocks shown in FIGS. 21 and 22 maybe skipped or omitted. In addition, any number of counters, statevariables, warning semaphores, or messages might be added to the logicalflow described herein, for purposes of enhanced utility, accounting,performance measurement, or providing troubleshooting aids, etc. It isunderstood that all such variations are within the scope of the presentdisclosure.

Disjunctive language such as the phrase “at least one of X, Y, or Z,”unless specifically stated otherwise, is otherwise understood with thecontext as used in general to present that an item, term, etc., may beeither X, Y, or Z, or any combination thereof (e.g., X, Y, and/or Z).Thus, such disjunctive language is not generally intended to, and shouldnot, imply that certain embodiments require at least one of X, at leastone of Y, or at least one of Z to each be present.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations setforth for a clear understanding of the principles of the disclosure.Many variations and modifications may be made to the above-describedembodiment(s) without departing substantially from the spirit andprinciples of the disclosure. All such modifications and variations areintended to be included herein within the scope of this disclosure andprotected by the following claims.

Therefore, the following is claimed:
 1. A sensor, comprising: a dryelectrode comprising a polymer material having a plurality of nanowiresdispersed therein; and a conductive element being attached to theelectrode.
 2. The sensor of claim 1, wherein the electrode is configuredto measure skin to electrode impedance.
 3. The sensor of claim 1,wherein a conductivity of the conductive element is retained during astrain of the sensor from 0% to 50%.
 4. The sensor of claim 1, whereinthe nanowires are silver nanowires.
 5. The sensor of claim 1, whereinthe nanowires are carbon nanotubes.
 6. The sensor of claim 1, whereinthe polymer material comprises a rubber substrate.
 7. The sensor ofclaim 6, wherein the rubber substrate comprises polydimethylsiloxane(PDMS).
 8. The sensor of claim 1, wherein the conductive elementcomprises a contact and a wire.
 9. The sensor of claim 1, wherein theelectrode and the conductive element are operatively connected tomedical equipment.
 10. The sensor of claim 9, wherein the medicalequipment is selected from a group consisting of electrocardiogram (ECG)equipment, electrocardiography (EKG) equipment, electroencephalogram(EEG) equipment, electromyogram (EMG) equipment, andimpedance-measurement equipment.
 11. The sensor of claim 1, wherein theelectrode and the conductive element are communicatively coupled to oneof photovoltaic equipment, a display device, and artificial skin. 12.The sensor of claim 1, wherein the electrode and the conductive elementare communicatively coupled to at least one of prosthetic equipment, amechanical motion detector, pressure sensing equipment, a strain gauge,hydration sensing equipment, a biomorph actuator, or an actuator.
 13. Amethod for creating a dry sensor, comprising: casting a plurality ofnanowires onto a substrate; pouring a liquid form ofpolydimethylsiloxane (PDMS) over the plurality of nanowires to create amixture of the plurality of nanowires and the PDMS; and pressing aconductive element on the mixture, the conductive element beingconfigured to communicatively couple to medical equipment.
 14. Themethod of claim 13, further comprising placing the substrate in a vacuumto remove air bubbles from the PDMS.
 15. The method of claim 13, furthercomprising curing the PDMS in an oven at 100° C. for 1 hour.
 16. Themethod of claim 13, further comprising peeling a cured portion of thePDMS off the substrate.
 17. The method of claim 13, wherein theplurality of nanowires are silver nanowires or carbon nanotubes.
 18. Themethod of claim 13, wherein the medical equipment is selected from agroup consisting of: electrocardiography (EKG) equipment,electroencephalogram (EEG) equipment, electromyogram (EMG) equipment,and impedance-measurement equipment.
 19. The method of claim 13, whereinthe conductive element is operatively connected to one of photovoltaicequipment, a display device, and artificial skin.
 20. The method ofclaim 13, wherein the conductive element is operatively connected to oneof prosthetic equipment, a mechanical motion detector, pressure sensingequipment, a strain gauge, hydration sensing equipment, a biomorphactuator, and an actuator.